Final answer:
Perfect competition is the market structure that is an ideal and doesn't exist in practice, serving as a benchmark for real-world imperfect markets like monopolistic competition, oligopoly, and monopoly.
Step-by-step explanation:
The market structure that is considered an ideal and does not really exist in the real world is perfect competition. This theoretical model assumes a number of conditions for its existence such as a large number of buyers and sellers, identical products sold by each company, and no barriers for entry or exit in the market. It's important to note that while perfect competition serves as a benchmark to measure other market structures against, it is purely hypothetical and not found in practice. In contrast, imperfect markets, which include monopolistic competition, oligopoly, and monopoly, are commonly observed. These imperfect markets lack some of the defining features of perfect competition and thus do not achieve the same level of efficiency.